Replies

1. 
   northhouguy | Oct 28, 2008 05:40am | #1

   What we call 120 volts and 240 volts are really Root Mean Square, (RMS) voltages.

   Alternating current in the US changes from 0 to a positive peak, back to 0, to a negative peak and back to 0 sixty times a second.  In other words, we have 60 Hertz power frequency in the US.  Don't say 60 cycles anymore.  That's old hat now.

   The peak of an AC wave is equal to the RMS voltage times the square root of two, (1.414)  So the peak of a 120 V wave is about 170 V, a 240 V wave about 340 V.  So the "peak to peak" voltage of 120 V RMS is 2 x 170, or about 340 V. Peak to peak of 240 V is 680 V.

   So, all AC line voltages that you would find in your shop are full sine waves that start at 0, reach a + peak, return to 0, reach a negative peak and return to 0, sixty time a second. 

   Shocking, isn't it!!??!! 

1. 
   Disputantum | Oct 28, 2008 06:07am | #3

   Mucho thankos, Northhouguy. It's a little more complicated than I understood.Can you recommend anything to read on the subject that might be suitable for a non-EE?Edit: thanks to Don Brown, too.

   Edited 10/27/2008 11:08 pm ET by Disputantum

1. 
   roc | Oct 28, 2008 07:25am | #5

   Books about electricity:Getting Started in Electronics by Forrest M. Mims, III Old Radio Shack book catalog No 276-5003 inexpensive paper back Very good Book ! ! !
   addresses alternating current p 18 and has totally helpful basic, basic, basic description of what electricity is and how it behaves and how we can manipulate/control it.Also get any really old mechanical physics book e.g., Elementary Practical Physics by Black and Davis 1939 The older books have better illustrations and are just more useful in general. See the Magnetism and Electricity section. One change/update since then is "conventional flow" verses "electron flow" no big deal but (some thing for you to investigate).I always tell kids that are looking at wizard books and occult etc. If they really want to study magic and wizard stuff to just look in a Practical Physics book or a Particle Physics book and to read about Richard Feynman !Some old Navy manuals that you can pick up at used book stores are fairly good.One book that is modern: Electrical / Electronic Systems that you may still be able to get at Grangers Supply. I looked for it on line but no luck. It was just in the purchasing area at the counter when I got mine.And one more; may still be available at Home Depot : Wiring Simplified by H.P. Richter and W.C. Schawan paper back.Edited 10/28/2008 12:29 am by rocEdited 10/28/2008 12:38 am by rocEdited 10/28/2008 12:42 am by rocEdited 10/28/2008 12:45 am by roc

   Edited 10/28/2008 1:00 am by roc

2. 
   DonaldCBrown | Oct 28, 2008 05:59am | #2

   Dis-

   Actually, 120v and 240v are not the peak amplitude for alternating current service. They are the rms (root mean square) values representing the equivalent of a steady DC voltage that would give the same heat when passed through a resistive load.

   Both 120v and 240v are pure sine waves (except for small noise voltages induced on the transmission line). The peaks of the 120v supply are positive and negative 170, and the 240 supply oscillates between plus and minus 340v.

   The transformer that serves your residence has a center-tapped secondary (the side that's coming to you), with the center grounded. Thus the voltages at the two hot ends of the transformer are 180 degrees out of phase and have nominal rms values of 120v. An AC voltmeter would read 120v from either end of the secondary to ground, and 240v if measuring between the ends.

   As with any sine wave, the passage through the zero point is instantaneous; there is no dwell at 0 volts at all.

   Don

1. 
   Disputantum | Oct 28, 2008 06:13am | #4

   Thus the voltages at the two hot ends of the transformer are 180 degrees out of phase and have nominal rms values of 120v. An AC voltmeter would read 120v from either end of the secondary to ground, and 240v if measuring between the ends.

   Is it still incorrect to call 240 AC "two-phase" then?

1. 
   JerryPacMan | Oct 28, 2008 03:17pm | #7

   Yes, 240 V AC it is sometimes called two-phase but that is incorrect.  The correct terminology is 3-wire, single-phase, mid-point neutral system.

   http://en.wikipedia.org/wiki/Split_phase 

   Life is what happens to you when you're making other plans .

   "Life is not a success only journey."  Dr. Phil

3. 
   Ray | Oct 28, 2008 07:31am | #6

   Picture incoming 3-phase power as the letter Y, each arm to the center is 120V, and from tip - to - tip is 240V.  The center point is neutral.  For a 240V. system, remove one of the arms from the Y.  Yes, there is a 120 degree electrical difference between the legs, that's what permits a 240V. motor to start without a capacitor - but that's really complicated :-)

1. 
   northhouguy | Oct 28, 2008 06:14pm | #8

   No, Ray, not correct.  In a single phase system one phase to ground is the phase to phase voltage divided by the square root of 3, (1.73).

   So, to derive a 120 V circuit from a Y connected three phase system the phase to phase voltage would be 208 V.

   If you had a 3 phase 240 V Y connected voltage source the phase to ground voltage would be about 139 V.

   We are speaking only of grounded Y systems here.  A delta connected system has no ground point.

1. 
   Disputantum | Oct 28, 2008 07:37pm | #9

   Thanks everyone for your replies. I'll definitely look for that Radio Shack book, Roc.

2. 
   Ray | Oct 28, 2008 08:05pm | #10

   OK, just trying to keep it somple :-)

4. 
   WillGeorge | Oct 29, 2008 08:46pm | #11

   Try this.. A 'bit complicated' but free reading.

   http://www.onlinefreeebooks.net/engineering-ebooks/electrical-engineering/basic-electrical-technology-pdf.html

   There is a TON of free information out on the Net for your question.

   Some, if not ALL, requires some basic math.

   This one is pretty good... Sort of easy to understand and has alot of illustrations.

   http://www.waterfront-woods.com/Articles/Electricity/electricity.htm

   If you REALLY want to know and have the time and money. Take a class at a local College night/day class. Great fun. Electrical engineer here. I go about every other year to see if anything has changed.. Nothing changed expept some really cool tools to 'see' what you have been taught.

5. 
   JLMCDANIEL | Oct 29, 2008 11:45pm | #12

   I don't know why delta a wye transformers are always brought up in these replies. Those transformers are for providing 3 phase electric  primarily for industrial areas, and substations on the grid. Almost all electricity for residential use is provided by a simple step down center tap transformer not a delta or wye transformer. The sine wave for 120 vac and the 240 vac is a full sine wave  going from 0 to +peak to 0 to -peak. The way to think about RMS voltage is that it is the average voltage for the full sine wave. Very few disciplines even need to know the actual peak voltage. By using the center tap to one side or the other of the transformer you are basically using half the voltage output.

   Jack

6. 
   larinUP | Oct 30, 2008 12:03am | #13

   Try,

   http://www.faqs.org/faqs/electrical-wiring/part1/preamble.html

   Larry